1. Field of the Invention
The present invention relates generally to acoustic structures that are used to attenuate noise that emanates from a particular source. More particularly, the present invention is directed to making contoured acoustic structures.
2. Description of Related Art
It is widely recognized that the best way of dealing with excess noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic damping structures (acoustic treatments) to the structure of the noise source. One particularly problematic noise source is the jet engine used on most passenger aircraft. Acoustic treatments are typically incorporated in the engine inlet, nacelle and exhaust structures. These acoustic treatments include acoustic resonators that contain relatively thin acoustic materials or grids that have millions of holes that create acoustic impedance to the sound energy generated by the engine.
Honeycomb has been a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight. For acoustic applications, such as engine nacelles, acoustic materials are added to the honeycomb structure so that the honeycomb cells are acoustically closed at the end located away from the engine and covered with a porous covering at the end located closest to the engine. The closing of the honeycomb cells with acoustic material in this manner creates an acoustic resonator that provides attenuation, damping or suppression of the noise. Acoustic septums are also usually incorporated into the interior of the honeycomb cells in order to provide the resonator with additional noise attenuation properties.
One way of incorporating acoustic septums into the honeycomb cells is to first form planar acoustic inserts from an acoustic material, such as an acoustic mesh or perforated acoustic film. The planar acoustic inserts are made larger than the cell openings. Accordingly, when the inserts are pushed into the cells with a plunger, they are folded into an acoustic septum in the form of a cap. The cap shape provides an anchoring portion that contacts the cell walls and a central septum portion which attenuates the sound waves in the cell. Once inserted into the cells, the friction between the anchoring portion of the acoustic septum cap and the honeycomb walls temporarily locks the acoustic septum cap in place. An adhesive is then used to permanently bond the anchoring portions of the inserted acoustic septum caps to the cell walls.
The permanent bonding of the acoustic septum caps is accomplished by dipping the entire honeycomb into a pool of liquid adhesive. The depth to which the honeycomb is dipped into the adhesive is chosen so that the anchoring portions of the inserted acoustic septum caps are immersed in the liquid adhesive. This adhesive dip process is particularly effective because it provides simultaneous bonding of the many hundreds of acoustic septums that are located within a typical honeycomb acoustic structure. An essential requirement of this procedure is that the honeycomb is planar or flat, so that the depth of immersion into the pool of adhesive is the same for all of the cells and is easily controlled. Acoustic honeycomb panels that utilize acoustic septum caps are described in detail in U.S. Pat. Nos. 7,434,659, 7,510,052, 7,854,298, 8,413,761, 8,579,076, 8,607,924 and 8,733,500, the contents of which are hereby incorporated by reference.
Many acoustic structures, such as jet engine nacelles, must be curved or contoured to provide a structure that surrounds the noise source. Curving or contouring of the flat acoustic honeycomb panels is typically accomplished by placing the flat panel on a curved mold surface. A combination of heat and pressure is then used to impart a curve to the panel that matches the mold surface. A number of smaller flat panels are typically assembled together to form a larger panel that is closer to the desired size of the final acoustic structure. The assembly of panels is contoured on a relatively large mold surface using heat and pressure. The assembled panels may be bonded together before, after or during formation of the contour on the mold
The above procedure of molding flat acoustic honeycomb panels is suitable for forming acoustic structures where the radius of the curve (radius of curvature) is relatively long. However, there are a number of situations, especially in forming jet engine nacelles, where is it desirable to reduce the radius of the curve to provide a panel with a tighter curve or contour. Attempts to use molding procedures to form acoustic honeycomb panels with relatively tight curves have failed. For example, the honeycomb walls and septa become distorted when the radius of the curve is reduced beyond conventional limits. In addition, the bond lines between the panels and the node bonds located within the panels are subject to failure as the radius of the curve is reduced.
There presently is a need to provide a method for forming contoured acoustic honeycomb panels in which the curvature of the panel is relatively tight while at the same time avoiding the problems of structural distortion, acoustic distortion and bond failure that have been associated with attempts to use conventional procedures to mold such tightly curved panels.